1. Field of the Invention
Embodiments of the present invention relates to a canister for measuring the natural gas content of rock cores. Embodiments of the present invention also relates to a procedure for using gas-sampling bags to periodically collect the gas released from the core.
2. Description of the Related Art
Coals and shales have long been known to contain substantial quantities of natural gas, but the low permeability of these rocks has prevented this gas from being utilized. However, with newer technologies such as horizontal drilling and controlled rock fracturing, and with increasing gas prices, it is now economic to produce gas from many coals and shales that were previously of no interest. Although current technology makes it possible to produce this gas, this technology is very expensive and thus it is imperative that the drilling company has information on the gas content in the target rocks before undertaking a very expensive well-completion project.
The conventional method for determining the “in-situ” gas content of coals and shales is to collect core samples using traditional drilling techniques, sealing those cores into core degassing canisters, and measuring the quantity and quality of gas released over time. The rate of gas release is then used to estimate the amount of gas that was lost between the time when the core was cut, and when it was sealed into the canister. This is termed “lost gas”. The total amount of recoverable in-situ gas in the rock is then assumed to be the sum of the lost gas and the released gas. Unfortunately, the amount of lost gas is sometimes a very significant fraction of the total and thus an accurate estimate of the lost gas is very important. Actually, those rocks which release their gas most readily are the ones of greatest economic interest, and the ones for which the accuracy of the lost gas volume is the most important, and the most difficult to estimate. The precision to which the lost gas can be estimated is very much dependant on the quality of data obtained by monitoring the degassing rate of the core samples in the canisters.
The canisters and procedures commonly used for core degassing studies are similar to those proposed by the US Bureau of Mines over 30 years ago. The typical degassing canister consists of a piece of aluminum pipe about 4″ in diameter and from one to two feet long (although different sizes are available for different sized cores). One end of the pipe has an end cap welded on, and the other end has a flange welded on that will allow bolting on a top, which consists of an aluminum disk of the same diameter as the flange, and sealed with a rubber gasket or o-ring. Generally this top cap has ports through it that allow attaching a valve, a pressure gauge, and sometimes a thermocouple.
When a core is drilled, as soon as it is received at the surface and extruded from the core barrel, workers must cut it into appropriate lengths and then transfer sections of the core to empty degassing canisters. As quickly as possible, the canister is sealed by applying the lid and then inserting and tightening the bolts evenly. The time when the canister is sealed is recorded. The buildup of pressure in the canister is monitored by watching the pressure gauge and provides a rough estimate of the amount of gas released.
Beginning a few minutes after the core is first sealed, and continued periodically thereafter (sometimes for several weeks), the gas is bled out of the canister and measured. The gas volume is generally measured by water displacement. One method consists of passing the gas into a water-filled graduated cylinder that is inverted in a pan of water such that the gas displaces the water in the cylinder. The amount of water displaced is then recorded to determine the amount of gas released. Another method involves displacing the water in a calibrated gas burette. Both of these methods generally involve using water displacement, which is very difficult during cold or freezing weather. The accuracy of these methods is very dependent on the calibration of the measurement equipment used and the skill of the person doing the measurements.
Several problems exist with current core-degassing canisters. Being custom made from welded aluminum or stainless steel, they are relatively expensive and are quite heavy. Also, because the dead-space around the core sample is generally air, the first samples taken from the canister may contain mostly air. To accurately determine what is being released from the core, the analysis of the sample must be mathematically corrected for the air derived from the dead space. The typical procedure for doing this is to analyze the oxygen in the sample, assume that all of that oxygen came from air contamination, and knowing that the ratio of nitrogen to oxygen in air is quite constant at 3.73, and that the ratio of oxygen to argon in air is 22.4, subtract off the oxygen and an appropriate amount of nitrogen and argon to determine the chemical makeup of an “air free” sample. One problem with this is that coal, and minerals in the coal, are highly reducing and will react with oxygen. Therefore, using oxygen to estimate the amount of air can result in greatly underestimating the amount of air contamination, and over estimating the amount of nitrogen in the released gas, which decreases its heating value. Some have attempted to use argon to make this correction, but this requires special analytical equipment and procedures that most gas analysis laboratories do not have. Also, coal gas and shale gas can contain some argon which is assumed to be entirely from air, resulting in over-estimation of the amount of air contamination. Some have tried to fill the canister with water to displace the air, but this ads several other problems such as providing an environment conducive to bacterial degradation of both the core material and the gas.
Another problem commonly encountered with existing core degassing equipment is chemical interaction between the gas and the material from which the canister is constructed. For example, most coal gas and shale gas contains carbon dioxide and water. When these two combine, they form carbonic acid. Carbonic acid can react with aluminum or steel (even stainless steel) to form a metal carbonate and hydrogen gas. Hydrogen that comes from a chemical reaction within the canister is not readily distinguished from hydrogen that occurs naturally within the rock. In some cases, the amount of hydrogen caused by reaction with the canister can exceed the amount of natural gas released from the coal and has a very negative impact on the released gas determination. Furthermore, some gas analysis laboratories do not have the capability to accurately measure hydrogen and so it is just ignored.